Optical codes, such as barcodes and other machine-readable indicia, appear in various places in a variety of applications. There are a variety of such optical codes, including: linear barcodes (e.g., UPC code), 2D codes including stacked barcodes (e.g., PDF-417 code), and matrix codes (e.g., Datamatrix code, QR code, or Maxicode).
There are several types of data readers used for reading these optical codes. The most common types of optical code readers are laser scanners and imaging readers. A laser scanner typically moves, i.e. scans, a laser light beam across the barcode. Imaging readers are typically used to capture a 2D image of an area, including the optical code or other scene, focused onto a detector array such as charge-coupled devices (CCDs) and complementary metal oxide semiconductor (CMOS) imagers. With some such imaging readers, it may be advantageous to provide a source of illumination that illuminates the optical code or other scene being imaged, to provide the required signal response in the imaging device. Such a source of illumination can reduce exposure time, thereby improving imager performance, especially in low ambient light conditions and when imaging moving items.
Businesses have begun sending optical codes to customers who display such optical codes on a portable electronic device, such as a mobile telephone, personal digital assistant, palm, tablet, or laptop computer, or other suitable device having an electronic display, such as a liquid crystal display (LCD). For example, an airline passenger may display an optical code on a portable electronic device for an airline employee to read using a data reader as verification of the passenger's ticket. In another application, a customer in a store may display an optical code on a portable electronic device for a cashier to read using a data reader to redeem a coupon. Optical codes are also included on other items having highly, or relatively highly, reflective surfaces, for example, but not limited to, identification (ID) cards, aluminum cans, and objects in plastic packaging.
Optical codes presented on, or under a highly or relatively highly reflective surface are typically difficult to decode using general-purpose data readers. In one solution to better read optical codes on LCD displays, the reader is cycled between reading modes, a first mode having the LED illumination on to illuminate the surface, and a second mode having the LED illumination off to avoid specular reflection.
Switching between the illumination and non-illumination modes results in the LED illumination to be cycled on and off producing an annoying/distracting light flickering as perceived by the user or bystander. For most people, light pulsed at a frequency in excess of 50 Hz will exceed their so-called “flicker fusion frequency” and no flicker effect will be perceived. The flicker fusion frequency of human vision typically ranges from as low as about 15 Hz to as high as about 65 Hz. To avoid flicker fusion, U.S. Pat. No. 7,234,641 discloses pulsing the light at a sufficiently high frequency (e.g., in excess of 50 Hz) to avoid the flicker effect.
The present inventors have, therefore, recognized a need for improved methods of pulsing illumination for a data reader. The present inventor has also recognized a need for methods of pulsing illumination in conjunction with operating an imaging reader, particularly methods for electronic display screens reading that avoid the flicker effect.